Abstract

The Opemisca and La Ronde Lake plutons are typical of post-Kenoran felsic plutons which intrude the Archean greenstone terrane of the eastern Abitibi orogen. Hornblendes from the plutons record undisturbed 40Ar/39Ar release spectra with total-gas ages of 2692 ± 30 and 2656 ± 30 m.y. (Opemisca) and 2650 ± 30 and 2623 ± 30 m.y. (La Ronde Lake). Two additional K–Ar ages of 2653 ± 64 and 2647 ± 61 m.y. have been determined for hornblendes from the Opemisca intrusive. Release spectra of biotite from the La Ronde Lake pluton are also undisturbed but define discordantly younger total-gas ages of 2544 ± 30 and 2559 ± 30 m.y. These are similar to a 2505 ± 30 m.y. 40Ar/39Ar total-gas age for biotite (undisturbed spectra) from the pre-Kenoran Opawica River pluton located immediately south of La Ronde Lake.It is suggested that the hornblende dates are close to the actual time of intrusion of both plutons. This range in age is similar to that of other post-kinematic plutons in the eastern Abitibi belt and suggests that Kenoran metamorphism must have occurred prior to ~2650–2700 m.y. Available age determinations for pre-kinematic plutons within the area are slightly older (~2780–2820 m.y.), and a bracket on the time of Kenoran metamorphism between 2650–2700 m.y. and 2780–2820 m.y. is defined.The 40Ar/39Ar biotite ages are similar to published K–Ar biotite ages (~2500–2550 m.y.) in the eastern Abitibi belt and are interpreted as dating the time during post-Kenoran metamorphic cooling when temperatures dropped below those required for argon retention in biotite.

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